If electronic and/or optoelectronic circuits housed in a package are operated at frequencies in the GHZ range, part of the microwave power may leak as electromagnetic radiation. The resulting electromagnetic waves are nearly completely reflected at the inner surfaces of metallic package walls. The package, depending on its internal dimensions, forms a resonant circuit which shows an undesirable resonance effect at given frequencies. Test measurements on a package designed to house electrical or optoelectronic devices have shown that at given frequencies, e.g., at approximately 2.5 GHz, 8 GHz, 12 GHZ, 17.5 GHZ, and 21 GHz, sharp resonances occur. This indicates that a considerable part of the microwave power to be transported into the device is coupled to a package resonance, which leads to drastically increased or reduced back reflection and thus causes correspondingly sharp variations in the power applied to the device. In circuits for digital or broadband analog applications, such power variations are not tolerable.